We are engaged in studying the hormonal and nonhormonal factors involved in the maternal behavior of the rat. Currently our research is focussing on interactions between estrogen and progesterone in the onset of maternal behavior, stimulus factors involved in the onset of maternal behavior, including olfactory and other stimuli from the pups, maternal cannibalism and its suppression by hormones, hormonal action on brain sites involved in maternal behavior, the role of oxytocin in estrogen-primed females and, in the near future, themoregulatory factors in maternal behavior and their control by hormones. We are searching for parallels in the action of estrogen in stimulating maternal behavior and its uptake in selected brain regions, the facilitating role of progesterone on estrogen behavioral effects and possible action on estrogen receptors in the brain. We are also engaged in preliminary studies on the hormonal basis of maternal behavior in hamsters and cats.